The present invention relates to apparatus in an electrostatographic copier or printer for removing unwanted image-defect causing particles from an imaging member which has a loose toner image thereon. More particularly, the present invention relates to such an apparatus that gently loosens and effectively removes substantially all such image-defect causing particles without adversely affecting the desired loose toner image remaining on the imaging member.
The process of producing or reproducing copies of images in an electrostatographic copier or printer involves moving an imaging member, in the form of a rigid drum or flexible web, past a series of stations. As this occurs, the imaging member is first charged, and then exposed to form a latent charged image thereon. The latent image is thereafter developed or made visible by moving it past a development station where charged, pigmented toner or particles of development material are attracted to and held by the latent image charges. The developed image is subsequently transferred, at a transfer station, to a suitable receiver, such as a copy sheet of paper which is thereafter advanced through a fusing station. At the fusing station, the toner particles forming the desired image on the copy sheet are heated and fused. Any particles remaining on the imaging member are thereafter removed, at a cleaning station, for example, using a cleaning fiber brush, prior to again charging the imaging member as the first step in repeating the process.
Despite such cleaning, some paper dust, pieces of brush fibers and other mostly airborne particles within the copier or printer, may thereafter be attracted to and remain on the imaging surface of the imaging member. Such particles usually result in image defects, if transferred, at the image transfer station, to the copy sheet of paper along with the desired image. As such, they are unwanted and should be removed prior to such image transfer. Additionally, other unwanted particles, which must also be similarly removed include carrier particles, usually ferromagnetic, which carry individual (wanted) particles of toner, and large agglomerated toner-toner particles or flakes of toner commonly present in development material. These latter particles are attracted to the latent image on the imaging member during development, along with individual wanted toner particles that will form the desired image. The carrier particles, which are heavier and larger than the toner particles they carry, ordinarily are not supposed to transfer as such to the latent image during development. The ones that unfortunately do transfer are therefore a problem because they cause image defects such as black spots, image voids and halftones, when transferred, at the transfer station, to the copy sheet along with the wanted, fine toner particles forming the desired image.
To attempt to prevent such image defects, various conventional apparatus have been developed for removing such unwanted particles from the image and/or non-image areas of the surface of the imaging member, prior to the desired image being transferred, at the transfer station, to the receiver or copy sheet. For example, a fixed magnet type scavenging apparatus is disclosed in commonly-assigned U.S. Pat. No. 3,543,720, which issued on Dec. 1, 1970 in the names of R. A. Drexler et al. Another example, which includes electrostatic charging, is disclosed in U.S. Pat. No. 4,435,073.
Additionally, it is also known to use positive air pressure apparatus for removal of toner-toner agglomerations or "tent poles" of toner material from the loose toner image area on a photoconductor. An example of this type of apparatus is disclosed in Item 24942 found on pages 73 and 74 of the January, 1985 edition of Research Disclosure, published by Kenneth Mason Publications Limited, the old Harbourmaster's, 8 North Street, Emsworth, Hampshire P.0. 10 7DD, England. A variation of this type of apparatus uses a vacuum or negative air pressure. See, for example, U.S. Pat. No. 4,014,065, issued Mar. 29, 1977 in the name of F. W. Hudson, which discloses a vacuum system for removing unwanted particles from the background area of a photoconductor.
In all such apparatus, there is a need to make the particular magnetic, electrostatic or pneumatic, particle removing component employed by the apparatus as strong as Possible, in order for it to be effective in removing the unwanted particles in a direction substantially normal to the surface of the imaging member. However, there is a limitation with making, for example, the electrostatic field or the pneumatic field alone as strong because each can then begin to destroy or adversely affect the loose toner image remaining on the imaging member. This limitation, together with the fact that the polarity and quantity of charges on all particles on the image and non-image areas of the imaging member are never fully known, have tended to substantially limit the effectiveness of such conventional scavenging apparatus.
Furthermore, since those Particles that are unwanted may be of the same polarity.(toner-toner or flakes), of opposite polarity (toner-carrier), or of unknown polarities (paper dust, brush fibers, etc.), a conventional scavenging apparatus employing only a vacuum effect, a fixed or slow moving magnetic field effect, or only the effect of a fixed single Polarity electrostatic field, will not sufficiently be effective in gently loosening and removing such Particles without also adversely affecting the desired image.